Nitrous oxide removal catalysts for exhaust systems

ABSTRACT

A nitrous oxide (N 2 O) removal catalyst composite is provided, comprising a N 2 O removal catalytic material on a substrate, the catalytic material comprising a rhodium (Rh) component supported on a ceria-based support, wherein the catalyst composite has a H 2 -consumption peak of about 100° C. or less as measured by hydrogen temperature-programmed reduction (H 2 -TPR). Methods of making and using the same are also provided.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a U.S. National Stage of International PatentApplication PCT/US2015/064484, filed Dec. 8, 2015 and claims priority toU.S. Provisional Patent Application No. 62/088,888, filed Dec. 8, 2014.The disclosures of each of the applications noted above are incorporatedherein by reference in their entirety.

FIELD OF THE INVENTION

The present invention is directed to a purifying catalyst for exhaustsystems of internal combustion engines, and methods for its use. Moreparticularly, the invention pertains to a catalyst comprising a rhodium(Rh) component supported on a ceria-based support, wherein the catalystis effective to remove nitrous oxide (N₂O) under conditions of anexhaust stream of an internal combustion engine. For example, the N₂Oremoval catalyst is effective to decompose N₂O to nitrogen (N₂) andoxygen (O₂) and/or to reduce N₂O to nitrogen and water and/or carbondioxide (depending on the reductant present).

BACKGROUND OF THE INVENTION

Nitrous oxide (N₂O) is a greenhouse gas with a global warming potentialof 310 times that of CO₂ and an atmospheric lifetime of 114 years.Automotive exhaust is one source of N₂O emissions, as a by-product offuel combustion and as a by-product formed during the catalyticreduction of nitrogen oxides (NO_(x)). N₂O is formed under transientconditions over all major classes of emission control catalysts,including three-way conversion (TWC) catalysts fortraditional/stoichiometric gasoline cars and for gasoline directinjection (GDI) gasoline cars, diesel oxidation catalysts (DOCs),catalyzed soot filters (CSFs), lean NO_(x) traps (LNTs), selectivecatalytic reduction catalysts (SCRs), which reduce NO, with urea, andselective ammonia oxidation catalysts (AMOx) catalysts for dieselvehicles.

Recognizing its global warming potential, US EPA has already set a N₂Oemission limit of 10 mg/mile for light-duty vehicles over the FTP cyclestarting from MY2012, and a N₂O emission limit of 0.1 g/bhp-h for heavyduty vehicles over the heavy duty FTP cycle starting from MY2014. In thepast, automobile catalyst systems were optimized for maximum reductionof NOx (a regulated pollutant) without accounting for N₂O level. Themore stringent regulations currently on N₂O emissions require that theemission control system design be optimized not only for high NOxconversion performance but also for low N₂O emissions. Under the presentstandards, if N₂O exceeds the 10 mg/mile limits, there is a penaltyagainst CAFE fuel economy requirements.

It is generally understood that N₂O can be decomposed industrially,e.g., in the context of treating off-gases from nitric acid and adipicacid production. The temperatures for these operations are much higher(>550° C., for example, about 800-900° C.) than the temperature oftypical automotive exhaust, and the process streams for these operationscontain little water (<1%), unlike typical exhaust gas streams. Thereare many literature reports describing N₂O decomposition catalysts, andmost can be grouped into three categories: (1) supported rhodium (Rh),(2) metal oxides with a spinel structure and (3) ion-exchanged zeolites.Such catalysts are usually in powder or pelleted form. InDE102008048159, decomposition of N₂O in a gas stream is conducted with acatalyst comprising rhodium supported on a gamma-alumina that isoptionally doped with cerium (Ce) or gold (Au).

KR20060019035 is directed to a method for removing nitrogen oxides usingdual catalyst beds, wherein nitrogen oxides are decomposed into nitrogenand nitrous oxide using a bed of nitrogen oxide-reducing catalystPt/V_(X)-P_(Y)-(material containing hydroxyl group)_(z), and the nitrousoxide thus formed is then further decomposed into nitrogen and oxideusing a bed of a nitrous oxide-decomposing catalyst comprising Rh andsilver (Ag), namely, Rh-Ag/CeO₂/M1-M2-M3, where M1 is magnesium (Mg),barium (Ba) or strontium (Sr), M2 is aluminum (Al), iron (Fe), vanadium(V), gallium (Ga) or chromium (Cr), and M3 is zinc (Zn), nickel (Ni), orcopper (Cu). There is a continuing need in the art to provide catalyticarticles that efficiently and effectively provide removal of nitrousoxide (N₂O), particularly under exhaust gas conditions.

SUMMARY OF THE INVENTION

The present disclosure generally provides catalyst compositions andcatalyst articles comprising such compositions. In particular, suchcompositions and articles can comprise catalysts for nitrous oxide (N₂O)reduction, e.g., from exhaust gas streams. Theoretically, reduction ofN₂O can be achieved by minimizing the formation of N₂O or by using acatalyst to decompose N₂O (e.g., by converting N₂O directly to N₂ and O₂and/or by converting N₂O to N₂ and H₂O and/or CO₂ (depending on thereductant)). Effective N₂O catalyst compositions and articles can beprovided as stand-alone materials or components or can be incorporatedinto existing catalyst systems.

A first aspect of the present disclosure provides a nitrous oxide (N₂O)removal catalyst for an exhaust stream of an internal combustion engine,comprising: a N₂O removal catalytic material on a substrate, thecatalytic material comprising a rhodium (Rh) component supported on aceria-based support, wherein the catalytic material has a H₂-consumptionpeak of about 100° C. or less (including 100° C. or less) as measured byhydrogen temperature-programmed reduction (H₂-TPR) and is effective todecompose nitrous oxide (N₂O) in the exhaust stream to nitrogen (N₂) andoxygen (O₂) and/or to reduce at least a portion of the N₂O to N₂ andwater, N₂ and carbon dioxide (CO₂) or N₂, water, and CO₂ underconditions of the exhaust stream.

In one embodiment, the H₂-consumption peak of aged catalytic material(e.g., after aging at 750° C. for 20 hours with 10 weight % water inair) occurs at a lower temperature than the temperature of theH₂-consumption peak of fresh catalytic material. In one or moreembodiments, N₂O removal activity of the catalytic material after agingat 750° C. for 20 hours with 10 weight % water is higher than N₂Oremoval activity of fresh catalytic material. In one or moreembodiments, the ceria-based support maintains about 90 to about 100% ofits pore volume after aging at 750° C. for 20 hours with 10 weight %water in air.

The ceria-based support may, in some embodiments, comprise about 90 toabout 100 weight % CeO₂ and have a pore volume that is at least about0.20 cm³/g. The ceria-based support may comprise a fresh surface areathat is in the range of about 40 to about 200 m²/g. The ceria-basedsupport may comprise a surface area that is in the range of about 20 toabout 140 m²/g after aging at 750° C. for 20 hours with 10 weight %water in air. The ceria may have an average crystallite size in therange of about 3 to about 20 nm measured by x-ray diffraction (XRD). Theceria-based support may comprise: an x-ray diffraction crystallite sizeratio of aged material to fresh material of about 2.5 or less, whereaging is 750° C. for 20 hours with 10% H₂O in air.

The ceria-based support may further comprise a promoter comprisingyttria, praseodymia, samaria, gadolinia, zirconia, or silica. Theceria-based support may comprise ceria in an amount in the range ofabout 56 to about 100% by weight of the support on an oxide basis.

The rhodium component may be present on the support in an amount in therange of about 0.01 to about 5% or even about 0.04 to about 3% by weightof the support. The rhodium component may have an average crystallitesize of less than about 5 nm. In some embodiments, the rhodium componentmay have an average crystallite size of about 3 to about 20 nm. Therhodium component may be loaded on the substrate in an amount in therange of about 1 to about 105 g/ft³.

The catalytic material may further comprise an additional metalcomponent. The additional metal component may comprise platinum (Pt),palladium (Pd), silver (Au), copper (Cu), or combinations thereof. Thecatalytic material may further comprise a metal oxide for promoting theRh and/or metal component(s). The metal oxide may comprise ceria,praseodymia, yttria, samaria, or gadolinia.

The substrate may comprise a monolithic substrate. Alternatively, thesubstrate may comprise a wall-flow filter.

Another aspect provides a catalyst composite for an exhaust stream of aninternal combustion engine comprising: a N₂O removal catalytic materialin a washcoat on a substrate, the catalytic material comprising arhodium (Rh) component supported on a ceria-based support and iseffective to convert nitrous oxide (N₂O) under conditions of the exhauststream, wherein the ceria-based support comprises: about 90 to about 100weight % CeO₂; a pore volume that is in the range of about 0.20 to about0.40 cm³/g; a fresh surface area that is in the range of about 40 toabout 200 m²/g; and an aged surface area that is in the range of about20 to about 140 m²/g after aging at 750° C. for 20 hours with 10 weight% water in air. The catalytic material may further include a preciousmetal on a high surface area refractory metal oxide support that iseffective to oxidize hydrocarbons and/or carbon monoxide underconditions of the exhaust stream.

Another aspect is an emissions treatment system for treatment of aninternal combustion engine exhaust stream including hydrocarbons, carbonmonoxide, and nitrogen oxides, the emission treatment system comprising:an exhaust conduit in fluid communication with the internal combustionengine via an exhaust manifold; a treatment catalyst; and any N₂Oremoval catalyst composite disclosed herein. The treatment catalyst maycomprise a nitrogen oxide treatment catalyst, which comprises: athree-way conversion (TWC) catalyst, a lean NOx trap (LNT), or aselective catalytic reduction (SCR) catalyst. The N₂O removal catalystcomposite may be located downstream of the nitrogen oxides treatmentcatalyst. The system may be zoned, wherein the nitrogen oxides treatmentcatalyst is in a front, upstream zone (i.e., where the gas flow enters)and the N₂O removal catalyst composite is in a back, downstream zone(i.e., where the gas flow exits). The system may be layered and thenitrogen oxides treatment catalyst is in an outer layer and the N₂Oremoval catalytic material of the catalyst composite is in an innerlayer. The system may be layered and the nitrogen oxides treatmentcatalyst is in an inner layer and the N₂O removal catalytic material ofthe N₂O catalyst composite is in an outer layer.

In a further aspect, provided is a method for treating exhaust gasescomprising contacting a gaseous stream comprising hydrocarbons, carbonmonoxide, and nitrogen oxides with any N₂O removal catalyst compositedisclosed herein. In an embodiment, N₂O removal activity of thecatalytic material after aging at 750° C. for 20 hours with 10 weight %water is higher than N₂O removal activity of fresh catalytic material.

Another aspect provides a method of making a nitrous oxide (N₂O) removalcatalyst composite, the method comprising: depositing a rhodiumprecursor onto a fresh ceria-based support having a pore volume that isat least about 0.20 cm³/g and forming a washcoat therefrom; coating asubstrate comprising a flow-through monolith or a wall-flow filter withthe washcoat to form a coated substrate; and calcining the coatedsubstrate at an elevated temperature. The calcining step may compriseaging, e.g., under conditions of 750° C. for 20 hours with 10 weight %water in air. In some embodiments, the method of making this catalystcomposite further comprises a step of calcining the fresh ceria-basedsupport at about 600° C. to about 800° C. before the depositing step.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosure may be more completely understood in consideration of thefollowing detailed description of various embodiments of the disclosurein connection with the accompanying drawings, in which:

FIG. 1 provides flow diagrams of exemplary diesel SCR exhaust systemsincluding N₂O catalysts;

FIG. 2 provides flow diagrams of exemplary diesel LNT exhaust systemsincluding N₂O catalysts;

FIG. 3 provides flow diagrams of exemplary gasoline TWC exhaust systemsincluding N₂O catalysts;

FIG. 4 provides flow diagrams of exemplary GDI exhaust systems includingN₂O catalysts;

FIG. 5 depicts an exemplary layered composite of SCR/AMOx catalyst and aN₂O catalyst;

FIG. 6 depicts an exemplary zoned composite of a SCR/AMOx catalyst and aN₂O catalyst;

FIGS. 7A-7D depict exemplary layered and zoned composites of an AMOxcatalyst and a N₂O catalyst;

FIG. 8 depicts an exemplary SCR catalyst with N₂O catalyst on awall-flow filter;

FIGS. 9A-9B depict exemplary wall-flow filters with multifunctionalcatalytic material including an N₂O catalyst;

FIGS. 10A-10B depict exemplary gasoline particulate filters having TWCcatalyst and N₂O catalyst;

FIG. 11 provides N₂O conversion versus temperature for an inventive N₂Ocatalytic material;

FIG. 12 provides N₂O conversion versus temperature for a comparative N₂Ocatalytic material;

FIG. 13 provides N₂O conversion versus temperature for an inventive N₂Ocatalyst composite; and

FIG. 14 provides N₂O conversion versus temperature for a comparative N₂Ocatalyst composite.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Before describing several exemplary embodiments of the invention, it isto be understood that the invention is not limited to the details ofconstruction or process steps set forth in the following description.The invention is capable of other embodiments and of being practiced orbeing carried out in various ways. Although the invention herein hasbeen described with reference to particular embodiments, it is to beunderstood that these embodiments are merely illustrative of theprinciples and applications of the present invention. It will beapparent to those skilled in the art that various modifications andvariations can be made to the method and apparatus of the presentinvention without departing from the spirit and scope of the invention.Thus, it is intended that the present invention include modificationsand variations that are within the scope of the appended claims andtheir equivalents.

Reference throughout this specification to “one embodiment,” “certainembodiments,” “one or more embodiments” or “an embodiment” means that aparticular feature, structure, material, or characteristic described inconnection with the embodiment is included in at least one embodiment ofthe invention. Thus, the appearances of phrases such as “in one or moreembodiments,” “in certain embodiments,” “in one embodiment” or “in anembodiment” in various places throughout this specification are notnecessarily referring to the same embodiment of the invention.Furthermore, the particular features, structures, materials, orcharacteristics may be combined in any suitable manner in one or moreembodiments. The articles “a” and “an” are used herein to refer to oneor to more than one (i.e., to at least one) of the grammatical object ofthe article. Any ranges cited herein are inclusive. The term “about”used throughout this specification is used to describe and account forsmall fluctuations. For example, the term “about” can refer to less thanor equal to ±5%, such as less than or equal to ±2%, less than or equalto ±1%, less than or equal to ±0.5%, less than or equal to ±0.2%, lessthan or equal to ±0.1% or less than or equal to ±0.05%. All numericvalues herein are modified by the term “about,” whether or notexplicitly indicated. A value modified by the term “about” of courseincludes the specific value. For instance, “about 5.0” must include 5.0.

Provided are N₂O catalyst compositions comprising one or more rhodium(Rh) components supported on a ceria-based support. In certainembodiments, selection of the ceria-based support may affect theactivity of the resulting catalyst composition, as will be describedmore fully herein. Advantageously, such catalyst compositions areeffective in removing at least a portion of the N₂O in various exhauststreams (e.g., mobile source exhaust streams).

The following definitions are used herein.

“H₂-TPR” refers to hydrogen temperature-programmed reduction, which isan analytical technique that measures the temperature at which acatalyst consumes H₂ under a set of defined conditions. Hydrogentemperature-programmed reduction (H₂-TPR) may be carried out on aMicromeritics AutoChem Series Instrument. Prior to the test, a sample ispretreated under a flow of 4% O₂ balanced with He at 500° C. for 30 minand then cooled down to ambient temperature. The TPR experiment is thenperformed by exposing the pretreated sample in 1% H₂ balanced with N₂ ata gas flow rate of 50 cc/min and the temperature is ramped from 20 to900° C. at a ramping rate of 10° C./min.

A platinum group metal (PGM) component refers to any compound thatincludes a PGM (Ru, Rh, Pd, Os, Ir, Pt and/or Au). For example, the PGMmay be in metallic form, with zero valance, or the PGM may be in anoxide form. Reference to “PGM component” allows for the presence of thePGM in any valance state. The terms “platinum (Pt) component,” “rhodium(Rh) component,” “palladium (Pd) component,” “iridium (Ir) component,”“ruthenium (Ru) component,” and the like refers to the respectiveplatinum group metal compound, complex, or the like which, uponcalcination or use of the catalyst, decomposes or otherwise converts toa catalytically active form, usually the metal or the metal oxide.

“BET surface area” has its usual meaning of referring to theBrunauer-Emmett-Teller method for determining surface area byN₂-adsorption measurements. Unless otherwise stated, “surface area”refers to BET surface area.

“Support” in a catalytic material or catalyst washcoat refers to amaterial that receives precious metals, stabilizers, promoters, binders,and the like through precipitation, association, dispersion,impregnation, or other suitable methods. Exemplary supports includerefractory metal oxide supports as described herein below.

“Refractory metal oxide supports” include bulk alumina, ceria, zirconia,titania, silica, magnesia, neodymia, and other materials known for suchuse, as well as physical mixtures or chemical combinations thereof,including atomically-doped combinations and including high surface areaor activated compounds such as activated alumina. Exemplary combinationsof metal oxides include alumina-zirconia, alumina-ceria-zirconia,lanthana-alumina, lanthana-zirconia-alumina, baria-alumina, barialanthana-alumina, baria lanthana-neodymia alumina, and alumina-ceria.Exemplary aluminas include large pore boehmite, gamma-alumina, anddelta/theta alumina. Useful commercial aluminas used as startingmaterials in exemplary processes include activated aluminas, such ashigh bulk density gamma-alumina, low or medium bulk density large poregamma-alumina, and low bulk density large pore boehmite andgamma-alumina, available from BASF Catalysts LLC (Port Allen, La., USA)and Sasol Germany GmbH (Hamburg, Germany). Such materials are generallyconsidered as providing durability to the resulting catalyst.

As used herein, the term “molecular sieves,” such as zeolites and otherzeolitic framework materials (e.g. isomorphously substituted materials),refers to materials that may, in particulate form, support catalyticprecious group metals. Molecular sieves are materials based on anextensive three-dimensional network of oxygen ions containing generallytetrahedral type sites and having a substantially uniform poredistribution, with the average pore size being no larger than 20 Å. Thepore sizes are defined by the ring size.

As used herein, the term “zeolite” refers to a specific example of amolecular sieve, further including silicon and aluminum atoms. Zeolitesare crystalline materials having rather uniform pore sizes which,depending upon the type of zeolite and the type and amount of cationsincluded in the zeolite lattice, range from about 3 to 10 Angstroms indiameter.

“High surface area refractory metal oxide supports” refer specificallyto support particles having pores larger than 20 Å and a wide poredistribution. High surface area refractory metal oxide supports, e.g.,alumina support materials, also referred to as “gamma alumina” or“activated alumina,” typically exhibit a BET surface area of freshmaterial in excess of 60 square meters per gram (“m²/g”), often up toabout 200 m²/g or higher. Such activated alumina is usually a mixture ofthe gamma and delta phases of alumina, but may also contain substantialamounts of eta, kappa and theta alumina phases.

“Rare earth metal oxides” refer to one or more oxides of scandium,yttrium, and the lanthanum series defined in the Periodic Table ofElements. Rare earth metal oxides can, in some embodiments, be bothexemplary oxygen storage components (OSCs) and promotes of oxygenstorage. Promoters are metals that enhance activity toward a desiredchemical reaction or function. Suitable promoters for oxygen storageinclude one or more rare earth metals selected from the group consistingof lanthanum, cerium, neodymium, gadolinium, yttrium, praseodymium,samarium, and mixtures thereof.

“Alkaline earth metal oxides” refer to Group II metal oxides, which areexemplary stabilizer materials. Suitable stabilizers include one or morenon-reducible metal oxides wherein the metal is selected from the groupconsisting of barium, calcium, magnesium, strontium and mixturesthereof. Preferably, the stabilizer comprises one or more oxides ofbarium and/or strontium.

“Washcoat” has its usual meaning in the art of a thin, adherent coatingof a material (e.g., a catalyst) applied to a substrate, such as ahoneycomb flow-through monolith substrate or a filter substrate which issufficiently porous to permit the passage therethrough of the gas streambeing treated. As used herein and as described in Heck, Ronald andRobert Farrauto, Catalytic Air Pollution Control, New York:Wiley-Interscience, 2002, pp. 18-19, a washcoat layer includes acompositionally distinct layer of material disposed on the surface of amonolithic substrate or an underlying washcoat layer. A substrate cancontain one or more washcoat layers, and each washcoat layer can haveunique chemical catalytic functions.

Reference to “monolithic substrate” means a unitary structure that ishomogeneous and continuous from inlet to outlet.

“Selective Catalytic Reduction” (SCR) is the catalytic reduction ofnitrogen oxides with a reductant in the presence of an appropriateamount of oxygen with the formation predominantly of nitrogen and steam.Reductants may be, for example, hydrocarbon, hydrogen, and/or ammonia.SCR reactions in the presence of ammonia occur according to thefollowing three reactions:4NO+4NH₃+O₂→4N₂+6H₂O,NO+NO₂+2NH₃→2N₂+3H₂O,6NO₂+8NH₄→7N₂+12H₂O.

“TWC” refers to the function of three-way conversion where hydrocarbons,carbon monoxide, and nitrogen oxides are substantially simultaneouslyconverted. A gasoline engine typically operates under nearstoichiometric reaction conditions that oscillate or are pertubatedslightly between fuel rich and fuel lean air to fuel ratios (A/F ratios)(λ=1±˜0.01). Use of “stoichiometric” herein refers to the conditions ofa gasoline engine, accounting for the oscillations or perturbations ofA/F ratios near stoichiometric. TWC catalysts include oxygen storagecomponents (OSCs) such as ceria that have multi-valent states whichallows oxygen to be held and released under varying air to fuel ratios.Under rich conditions when NOx is being reduced, the OSC provides asmall amount of oxygen to consume unreacted CO and HC. Likewise, underlean conditions when CO and HC are being oxidized, the OSC reacts withexcess oxygen and/or NOx. As a result, even in the presence of anatmosphere that oscillates between fuel rich and fuel lean air to fuelratios, there is conversion of HC, CO, and NOx all at the same (or atessentially all the same) time.

Typically, a TWC catalyst comprises one or more platinum group metalssuch as palladium and/or rhodium and optionally platinum; an oxygenstorage component; and optionally promoters and/or stabilizers. Underrich conditions, TWC catalysts can generate ammonia.

“OSC” refers to an oxygen storage component, which is an entity that hasmulti-valent oxidation states and can actively react with oxidants suchas oxygen (O₂) or nitric oxides (NO₂) under oxidative conditions, orreacts with reductants such as carbon monoxide (CO), hydrocarbons (HC),or hydrogen (H₂) under reduction conditions. Examples of suitable oxygenstorage components include ceria. Praseodymia can also be included as anOSC. Delivery of an OSC to the washcoat layer can be achieved by the useof, for example, mixed oxides. For example, ceria can be delivered as amixed oxide of cerium and zirconium, and/or a mixed oxide of cerium,zirconium, and neodymium. For example, praseodymia can be delivered as amixed oxide of praseodymium and zirconium, and/or a mixed oxide ofpraseodymium, cerium, lanthanum, yttrium, zirconium, and neodymium.

“DOC” refers to a diesel oxidation catalyst, which converts hydrocarbonsand carbon monoxide in the exhaust gas of a diesel engine. Typically, aDOC comprises one or more platinum group metals such as palladium and/orplatinum; a support material such as alumina; zeolites for HC storage;and optionally promoters and/or stabilizers.

“LNT” refers to a lean-NOx trap, which is a catalyst containing aplatinum group metal, ceria, and an alkaline earth trap materialsuitable to adsorb NOx during lean conditions (for example, BaO or MgO).Under rich conditions, NOx is released and reduced to nitrogen.

“CSF” refers to a catalyzed soot filter, which is a wall-flow monolithhaving an oxidation catalyst suitable to collect soot particles at lowtemperature and to burn soot during regeneration conditions.

“GDI” refers to a gasoline direct injection gasoline engine, whichoperates under lean burn conditions.

“AMOx” refers to a selective ammonia oxidation catalyst, which is acatalyst containing one or more metals (typically Pt, although notlimited thereto) and an SCR catalyst suitable to convert ammonia tonitrogen.

Catalyst Compositions:

As noted above, the catalyst compositions generally disclosed hereincomprise a Rh component on a ceria-based support (e.g., impregnated on aceria based support). By “ceria-based support” is meant a material(e.g., a refractory metal oxide support material) comprising at leastabout 50% by weight ceria. In certain embodiments, the ceria-basedsupport comprises at least about 55 wt. %, at least about 60 wt. %, atleast about 65 wt. %, at least about 70 wt. %, at least about 75 wt. %,at least about 80 wt. %, at least about 85 wt. %, at least about 90 wt.%, at least about 91 wt. %, at least about 92 wt. %, at least about 92wt. %, at least about 93 wt. %, at least about 94 wt. %, at least about95 wt. %, at least about 96 wt. %, at least about 97 wt. %, at leastabout 98 wt. %, at least about 99 wt. %, or even at least about 99.9 wt.% ceria (based on the entire weight of the ceria-based support). In someembodiments, the ceria-based support may comprise, in addition to ceria,various metal oxides (resulting in a mixed metal oxide compositesupport). Exemplary metal oxides that may be included in the ceria-basedsupport include zirconia, lanthana, yttria, praseodymia, neodymia,samaria, gadolinia, or other rare earth metal oxides.

Advantageously, in certain embodiments, the ceria-based support is atleast about 90% by weight ceria or at least about 95% by weight ceria,and in some embodiments, about 100% by weight ceria. In someembodiments, the ceria-based support can be described as consisting ofceria or consisting essentially of ceria. The ceria-based support can,in some embodiments, be described as being substantially free of othermetal oxides. Ceria-based supports can, in some embodiments, bedescribed as being highly stable. By “highly stable” in this context ismeant that the decrease in BET surface area is less than about 60% andthe decrease in pore volume is less than about 10% after the material iscalcined at 750° C. for 20 hours with 10% water/steam in air.

Preparation of N₂O Catalyst Compositions:

N₂O catalyst compositions comprising a rhodium component and aceria-based support according to the present disclosure are commonlyprovided in the form of a washcoat, and such washcoats may be made byvarious techniques. The preparation of the catalyst compositiongenerally comprises impregnating the ceria-based support in particulateform with a solution comprising a rhodium reagent. For the purposesherein, the term “rhodium reagent” means any rhodium-containingcompound, salt, complex, or the like which, upon calcination or usethereof, decomposes or otherwise converts to a catalytically active form(i.e., rhodium component), usually the metal or the metal oxide (i.e.,rhodium or rhodium oxide).

As referenced herein above, selection of the (fresh) ceria-based supportupon which the rhodium reagent is impregnated may affect the activity ofthe catalyst compositions produced according to the present disclosure.In some embodiments, analysis of a given fresh ceria-based support isuseful in predicting the activity of a catalyst composition according tothe present disclosure. See Example 2, below, for various physical andchemical characterization methods that can be employed for this purpose.Relevant considerations can include, but are not limited to, porevolumes, surface areas (e.g., BET surface areas), average pore radius,and x-ray diffraction patterns of the fresh ceria-based support. Porevolumes of certain preferred fresh ceria-based supports are usually atleast about 0.20 cm³/g. In certain embodiments, the pore volume of thefresh ceria-based supports is in the range of about 0.20 to 0.40 cm³/g.Surface areas of certain preferred fresh ceria supports are generally atleast about 40 m²/g and in some embodiments, may be at least about 60m²/g, at least about 80 m²/g, or at least about 100 m²/g. In certainembodiments, surface areas of the fresh ceria-based supports are in therange of about 40 to about 200 m²/g, and in some embodiments, in therange of about 100 to about 180 m²/g.

Although not intending to be limited by theory, it is believed thatparticular ceria-based supports are more effective at absorbing O₂. Itis generally understood that catalytic decomposition of N₂O consists ofthe following steps (Equations 1 to 4), where

is a catalytically active siteN₂O+

→N₂O−

  (1)N₂O−

→N₂+O−

  (2)2O−

→O₂+2

  (3)N₂O+O−→

→N₂+O₂+

  (4)

A N₂O molecule contacts with an active site and adsorbs thereon(Reaction 1). The adsorbed N₂O is then dissociated to N₂ and an adsorbedO atom (Reaction 2). Two adsorbed O atoms can combine to form an O₂molecule, restoring the open active sites (Reaction 3). Alternatively, aN₂O molecule can react with an adsorbed O atom, producing O₂ and N₂(Reaction 4). On a metal surface, Reaction 2 can readily take place atroom temperature. However, the O₂ formation step (Reaction 3) is moredifficult because of strong O-metal bonding and requires hightemperatures (typically >600° C.). As the metal surface is beingoxidized by the decomposition product (oxygen), the decompositionreaction slows down and eventually stops. Therefore, the ratedetermining step for a metal catalyst is usually the oxygen formationstep. Under oxidizing conditions, most metals are not stable andconverted to oxides. On an oxide catalyst, the N₂O decomposition step(Reaction 2) becomes critical too. Although not intending to be limitedby theory, it is believed that the surprising activity of the Rh/CeO₂catalyst may be attributed to its ability to decompose N₂O and to formO₂. The active state of Rh is Rh oxide as detected by X-rayphotoelectron spectroscopy. It is speculated that the O atoms, formed asresult of N₂O decomposition, can be channeled away from the Rh sites tothe CeO₂ support, where they combine to form O₂. The special capabilityof the CeO₂ in promoting oxygen mobility is believed to be a key forsustained N₂O decomposition activity.

In general terms, the rhodium reagent (e.g., in the form of a rhodiumsalt solution) can be impregnated onto a ceria-based support (e.g.,powder) by, for example, incipient wetness techniques. Water-solublerhodium compounds or salts or water-dispersible compounds or complexesof the metal component may be used as long as the liquid medium used toimpregnate or deposit the metal component onto the support particlesdoes not adversely react with the metal or its compound or its complexor other components which may be present in the catalyst composition andis capable of being removed from the Rh reagent/Rh component byvolatilization or decomposition upon heating and/or application of avacuum. In some cases, the completion of removal of the liquid may nottake place until the catalyst is placed into use and subjected to thehigh temperatures encountered during operation. Generally, both from thepoint of view of economics and environmental aspects, aqueous solutionsof soluble compounds, salts, or complexes of rhodium are advantageouslyutilized as Rh reagents.

The impregnated powder is then slurried in deionized water to form awashcoat. Additional process steps may be applied to either theimpregnated powder or the slurry prior to coating the washcoat onto asubstrate. In some embodiments, desired additional ingredients such asother platinum group metals, stabilizers, and/or promoters can be addedto the slurry.

In one or more embodiments, the slurry is acidic, having a pH of about 2to less than about 7. The pH of the slurry may be lowered by theaddition of an adequate amount of an inorganic or an organic acid to theslurry. Combinations of both an inorganic acid and an organic acid canbe used in some embodiments, particularly when compatibility of acid andraw materials is considered. Inorganic acids include, but are notlimited to, nitric acid. Organic acids include, but are not limited to,acetic acid, propionic acid, oxalic acid, malonic acid, succinic acidglutamic acid, adipic acid, maleic acid, fumaric acid, phthalic acid,tartaric acid, citric acid and the like and combinations thereof.Thereafter, if desired, water-soluble or water-dispersible compounds ofa stabilizer, e.g., barium acetate, and/or a promoter, e.g., lanthanumnitrate, may be added to the slurry.

In some embodiments, the slurry may thereafter be comminuted to resultin substantially all of the solids having particle sizes of less than agiven size, e.g., less than about 20 microns average diameter, such asbetween about 0.1-15 microns average diameter (for example, for coatingonto a substrate/carrier that is a flow-through monolith). The optionalcomminution may be accomplished in a ball mill or other similarequipment, and the solids content of the slurry may be, e.g., about10-50 wt. %, more particularly about 10-40 wt. % for coating onto aflow-through monolith. In some embodiments, the slurry may be comminutedto result in substantially all of the solids having particle sizes ofless than about 10 microns average diameter, such as between about 2-3microns average diameter (for example, for coating onto asubstrate/carrier that is a wall flow monolith). The optionalcomminution may be accomplished in a ball mill or other similarequipment, and the solids content of the slurry may be, e.g., about10-50 wt. %, more particularly about 10-40 wt. % for coating onto aflow-through monolith and, e.g., about 5-30 wt. %, more particularlyabout 10-20 wt. % for coating onto a wall flow monolith. Thereafter theRh-impregnated ceria-based support is generally calcined. An exemplarycalcination process involves heat treatment in air at a temperature ofabout 400 to about 800° C. for about 10 minutes to about 3 hours. Duringthe calcination step, or at least during the initial phase of use of thecatalytic composition, the Rh reagent is converted into a catalyticallyactive form of the metal or a compound thereof. The above process can berepeated as needed to reach the desired level of Rh impregnation. Theresulting material can be stored as a dry powder or in slurry form.

Substrate:

According to one or more embodiments, the substrate for the N₂O catalystcomposition disclosed herein may be constructed of any materialtypically used for preparing automotive catalysts and will typicallycomprise a metal or ceramic honeycomb structure. The substrate typicallyprovides a plurality of wall surfaces upon which a washcoat comprisingthe N₂O catalyst composition is applied and adhered, thereby acting as acarrier for the catalyst composition. The catalytic material istypically disposed on a substrate such as a monolithic substrate forexhaust gas applications.

Any suitable substrate may be employed, such as a monolithic substrateof the type having fine, parallel gas flow passages extendingtherethrough from an inlet or an outlet face of the substrate, such thatpassages are open to fluid flow therethrough (referred to as honeycombflow through substrates). The passages, which are essentially straightpaths from their fluid inlet to their fluid outlet, are defined by wallson which the catalytic material is coated as a washcoat so that thegases flowing through the passages contact the catalytic material. Theflow passages of the monolithic substrate are thin-walled channels,which can be of any suitable cross-sectional shape and size such astrapezoidal, rectangular, square, sinusoidal, hexagonal, oval, circular,etc. Such structures may contain from about 60 to about 900 or more gasinlet openings (i.e., cells) per square inch of cross section.

The substrate can also be a wall-flow filter substrate, where thechannels are alternately blocked, allowing a gaseous stream entering thechannels from one direction (inlet direction), to flow through thechannel walls and exit from the channels from the other direction(outlet direction). A dual oxidation catalyst composition can be coatedon the wall-flow filter. If such a substrate is utilized, the resultingsystem will be able to remove particulate matters along with gaseouspollutants. The wall-flow filter substrate can be made from materialscommonly known in the art, such as cordierite or silicon carbide.

The substrate may be made of any suitable refractory material, e.g.,cordierite, cordierite-alumina, silicon nitride, zircon mullite,spodumene, alumina-silica magnesia, zircon silicate, sillimanite, amagnesium silicate, zircon, petalite, alumina, an aluminosilicate andthe like.

The substrates useful for the catalysts of the present invention mayalso be metallic in nature and be composed of one or more metals ormetal alloys. The metallic substrates may be employed in various shapessuch as corrugated sheet or monolithic form. Preferred metallic supportsinclude the heat resistant metals and metal alloys such as titanium andstainless steel as well as other alloys in which iron is a substantialor major component. Such alloys may contain one or more of nickel,chromium and/or aluminum, and the total amount of these metals mayadvantageously comprise at least 15 wt. % of the alloy, e.g., 10-25 wt.% of chromium, 3-8 wt. % of aluminum and up to 20 wt. % of nickel. Thealloys may also contain small or trace amounts of one or more othermetals such as manganese, copper, vanadium, titanium and the like. Thesurface of the metal substrates may be oxidized at high temperatures,e.g., 1000° C. and higher, to improve the resistance to corrosion of thealloys by forming an oxide layer on the surfaces of thesubstrates/carriers. Such high temperature-induced oxidation may enhancethe adherence of the refractory metal oxide support and catalyticallypromoting metal components to the substrate.

In alternative embodiments, one or more catalyst compositions may bedeposited on an open cell foam substrate. Such substrates are well knownin the art, and are typically formed of refractory ceramic or metallicmaterials.

Coating the Substrate:

The above-noted catalyst composition, in the form of ceria-based supportparticles impregnated with a Rh reagent/component, is mixed with waterto form a slurry for purposes of coating a catalyst substrate, such asthe types of substrate described herein above. In addition to thecatalyst particles, the slurry may optionally contain alumina as abinder, hydrocarbon (HC) storage components (e.g., zeolite),water-soluble or water-dispersible stabilizers (e.g., barium acetate),promoters (e.g., lanthanum nitrate), associative thickeners, and/orsurfactants (including anionic, cationic, non-ionic or amphotericsurfactants).

The slurry can be milled to enhance mixing of the particles andformation of a homogenous material. The milling can be accomplished in aball mill, continuous mill, or other similar equipment to ensure solidscontents, e.g., within the ranges noted above with respect to coatingvarious types of substrates.

The slurry is then coated on the catalyst substrate using a washcoattechnique known in the art. As used herein, the term “washcoat” has itsusual meaning in the art of a thin, adherent coating of a catalyticmaterial applied to a substrate. In one embodiment, the catalystsubstrate is dipped one or more times in the slurry or otherwise coatedwith the slurry. In some embodiments, the catalyst slurry is applied tothe substrate such that a desired loading of the washcoat is deposited,e.g., about 0.5 to about 3.0 g/in³.

Thereafter, the coated substrate is dried at an elevated temperature(e.g., about 100 to about 150° C.) for a period of time (e.g., about 1to about 3 hours) and then calcined by heating, e.g., at about 400 toabout 600° C., typically for about 10 minutes to about 3 hours.Following drying and calcining, the final washcoat coating layer cangenerally be viewed as essentially solvent-free.

After calcining, the catalyst loading can be determined throughcalculation of the difference in coated and uncoated weights of thesubstrate. As will be apparent to those of skill in the art, thecatalyst loading can be modified by altering the slurry rheology. Inaddition, the coating/drying/calcining process can be repeated as neededto build the coating on the substrate to the desired loading level orthickness. The catalyst composition can be applied as a single layer orin multiple layers to the substrate.

In certain embodiments, the coated substrate is aged, by subjecting thecoated substrate to heat treatment, e.g., at 750° C., 10 wt. % water inair, 20 hours aging. Aged catalyst articles are thus provided in certainembodiments. In certain embodiments, particularly effective materialscomprise ceria-based supports (including, but not limited tosubstantially 100% ceria supports) that maintain a high percentage(e.g., about 95-100%) of their pore volumes upon aging (e.g., at 750°C., 10 wt. % water in air, 20 hours aging). Accordingly, pore volumes ofaged ceria-based supports can be, in some embodiments, at least about0.18 cm³/g, at least about 0.19 cm³/g, or at least about 0.20 cm³/g,e.g., about 0.18 cm³/g to about 0.40 cm³/g.

The surface areas of aged ceria supports (e.g., after aging at theabove-noted conditions) can be, for example, within the range of about20 to about 140 m²/g (e.g., based on aging fresh ceria supports havingsurface areas of about 40 to about 200 m²/g) or about 50 to about 100m²/g (e.g., based on aging fresh ceria supports having surface areas ofabout 100 to about 180 m²/g). Accordingly, surface areas of preferredaged ceria-based supports are in the range of about 50 to about 100 m²/gafter aging at 750° C. for 20 hours with 10 weight % water in air. Insome embodiments, the fresh and aged material can be analyzed by x-raydiffraction, wherein, for example, the average crystallite size ratio offresh to aged catalyst article can be about 2.5 or less, where aging isat the above-noted conditions.

Catalyst Activity:

Catalysts as disclosed herein are effective to decompose at least aportion of the nitrous oxide (N₂O) in exhaust gas to nitrogen (N₂) andoxygen (O₂) and/or to reduce at least a portion of the N₂O therein toN₂, H₂O, and/or CO₂ (depending on the reductant) under conditions ofexhaust streams of various types of internal combustion engines. By “atleast a portion” is meant some percentage of the total N₂O in theexhaust gas stream is decomposed and/or reduced. For example, in someembodiments, at least about 1%, at least about 2%, at least about 5%, atleast about 10%, at least about 20%, at least about 30%, at least about40%, at least about 50%, at least about 60%, at least about 70%, atleast about 80%, or at least about 90% by weight of the nitrous oxide inthe gas stream is decomposed and/or reduced under such conditions. Notethat actvity is dependent on reaction temperature.

For example, under lean conditions, such as those found in exhauststreams of diesel engines operating with a SCR catalyst, a N₂O catalystas described herein can decompose N₂O present in the exhaust stream.Under oscillating conditions, such as those found in exhaust streams ofdiesel and/or gasoline direct injection (GDI) engines operating with anLNT or a TWC, a N₂O catalyst as described herein can reduce N₂Oformation.

The conversion chemistries follow the following reactions:

Decomposition: 2N₂O→2N₂+O₂ (I)

Reduction:N₂O+H₂→N₂+H₂O  (IIa)N₂O+HC→N₂+CO₂+H₂O  (IIb)N₂O+CO→N₂+CO₂  (IIc)3N₂O+2NH₃→4N₂+3H₂O  (IId).

It is generally understood that the N₂O conversion activity of acatalyst is related to its reducibility, i.e. the ability to loseoxygen. One measure of reducibility is H₂-TPR, hydrogentemperature-programmed reduction, which measures the temperature atwhich the catalyst consumes H₂ under a set of defined conditions. Thelower the temperature of the H₂-consumption peak, the more readilyreducible the catalyst and the more active it is. In one or moreembodiments, the catalyst demonstrates a H₂-peak of 100° C. or lessunder fresh and/or aged conditions.

The disclosed rhodium-containing catalysts, comprising a highly stableceria-based support, (for example, a support whose majority content isceria), can provide a highly active catalyst for decomposing N₂O atrelatively low temperatures, such as those of engine exhaust (forexample, about 300 to 500° C.). The ceria-based supports used herein arehighly compatible with rhodium. It is believed that the interaction(and, in some embodiments, possible synergy) between rhodium and theceria-based supports increases the mobility of oxygen in the catalystand therefore promotes the catalytic activity for N₂O decomposition. Itwas unexpectedly found that this promotion effect was much morepronounced after a thermal treatment (i.e., aging) of the Rh catalyst athigh temperatures, for example, 600° C. to about 800° C., such as around750° C. for at least 10 hours (e.g., about 10 to about 30 hours, e.g.,about 20 hours) or by pre-calcining the ceria-containing support beforeRh impregnation at high temperatures, e.g., 600° C. to about 800° C.,such as around 700° C. for at least 1 hour (e.g., about 1 to about 5hours, about 1 to about 3 hours, or about 2 hours). This finding issurprising, as it is in clear contrast to other catalyst compositions,where high temperature aging has a detrimental effect on their catalystactivities (generally due to catalyst sintering under such conditions).

Emission Treatment System

The present invention also provides an emission treatment system thatincorporates the N₂O conversion catalyst composition or articledescribed herein. The N₂O conversion catalyst composition of the presentinvention is typically used in an integrated emissions treatment systemcomprising one or more additional components for the treatment of dieselexhaust gas emissions. As such, the terms “exhaust stream”, “engineexhaust stream”, “exhaust gas stream” and the like refer to engineeffluent as well as to the effluent downstream of one or more othercatalyst system components as described herein.

N₂O catalyst compositions and articles as disclosed herein may beincorporated in various ways within an exhaust gas stream system. TheN₂O catalyst compositions disclosed herein may be provided in in someembodiments, in the form of a catalytic article comprising the N₂Ocatalyst, substantially free of other catalytic material. In otherembodiments, the N₂O catalyst compositions can be provided on an articlewith other catalytic materials (e.g., giving a catalytic articlecomprising two or more catalytic compositions). As such, the N₂O removalcatalysts may be used independently or in conjunction with othercatalytically active materials in any combination (including but notlimited to, configurations such as a homogeneous mixture, a zonedstructure, and/or a layered structure). For example, the N₂O catalyst(i.e., a rhodium component on a ceria-based support) may be used inconjunction with another precious metal (e.g., Pt and/or Pd) on a highsurface area refractory metal oxide support (e.g., γ-Al₂O₃) that iseffective to oxidize hydrocarbons and/or carbon monoxide underconditions of the exhaust stream. Such an overall combination ofcatalytic materials may in turn be used to formulate an AMOx catalyst,an LNT catalyst, and/or a TWC catalyst with the optional addition offurther components such as other precious metals, supports, stabilizers,promoters, binders, and the like. Additional functional catalytic layersmay be prepared and deposited upon previous layers in the same manner asdescribed above for deposition of any layer upon the support.

N₂O catalyst compositions and/or articles are generally employed incombination with one or more other components (e.g., other catalysts, aswill be described in greater detail below). The N₂O catalyst can bepositioned upstream or downstream from such other components.“Downstream” refers to a position of a component in an exhaust gasstream in a path further away from the engine than the precedingcomponent. For example, when a diesel particulate filter is referred toas downstream from a diesel oxidation catalyst, exhaust gas emanatingfrom the engine in an exhaust conduit flows through the diesel oxidationcatalyst before flowing through the diesel particulate filter. Thus,“upstream” refers to a component that is located closer to the enginerelative to another component. In all flow diagrams referenced herein,the gas flow is understood to be from left to right such that, forexample, in FIGS. 1-4, the N₂O catalyst (shown as “N₂O”) is generallydownstream of the other components shown in the illustrated systems,although the invention is not intended to be limited thereto.

For example, within a diesel exhaust gas stream system, an N₂O catalystis generally positioned downstream of a diesel oxidation catalyst (DOC).Turning to the figures, FIG. 1 provides flow diagrams of exemplarydiesel SCR exhaust systems including N₂O catalysts. System 1A depicts aDOC followed by a catalyzed soot filter (CSF) followed by a urea feedfor a selective catalytic reduction (SCR) catalyst, followed by a N₂Ocatalyst. System 1B depicts a DOC followed by a urea injector and an SCRcatalyst, followed by a CSF and a N₂O catalyst. System 1C depicts a DOCfollowed by a urea injector a SCR catalyst on a filter (SCRoF) followedby a N₂O catalyst. System 1D depicts a DOC followed by a urea injector aSCR catalyst on a filter (SCRoF) followed by a combination N₂O catalystand ammonia oxidation (AMOx) catalyst. System 1E depicts a DOC followedby a urea injector and a combination filter design with both SCRcatalyst on a filter (SCRoF) and N₂O catalyst.

FIG. 2 provides flow diagrams of exemplary diesel LNT exhaust systemsincluding N₂O catalysts. System 2A depicts a lean NOx trap (LNT)followed by a CSF followed by a N₂O catalyst. System 2B depicts a LNTfollowed by a N₂O catalyst on a catalyzed soot filter (CSF+N₂O). System2C depicts a LNT followed by a CSF, followed by a SCR catalyst, followedby a N₂O catalyst. System 2D depicts a LNT followed by a SCR catalyst ona filter (SCRoF) followed by a N₂O catalyst. System 2E depicts a LNTfollowed by a N₂O catalyst on a SCRoF.

FIG. 3 provides flow diagrams of exemplary gasoline TWC exhaust systemsincluding N₂O catalysts. System 3A depicts a three-way conversion (TWC)catalyst followed by a N₂O catalyst. System 3B depicts a TWC catalystfollowed by another catalyst in which the second/rear TWC also containsa N₂O catalyst. System 3C depicts a TWC followed by a gasolineparticulate filter (GPF) followed by an N₂O catalyst. System 3D depictsa TWC followed by a N₂O catalyst on a gasoline particulate filter(GPF+N₂O).

FIG. 4 provides flow diagrams of exemplary GDI exhaust systems includingN₂O catalysts. System 4A depicts a three-way conversion (TWC) catalystfollowed by a LNT followed by a N₂O catalyst. System 4B depicts a TWCfollowed by a LNT followed by a gasoline particulate filter (GPF),followed by a N₂O catalyst. System 4C depicts a TWC followed by a LNT,followed by a N₂O catalyst on a gasoline particulate filter (GPF+N₂O).System 4D depicts a TWC followed by a LNT followed by a SCR catalystfollowed by a N₂O catalyst. System 4E depicts a TWC followed by a LNTfollowed by a SCRoF, followed by a N₂O catalyst. System 4F depicts a TWCfollowed by a LNT, followed by a N₂O catalyst on a SCRoF.

FIG. 5 depicts an exemplary layered composite 50 of an SCR/AMOx catalystand a N₂O catalyst according to the present disclosure where a top layer56 comprises a Cu-zeolite catalyst for SCR and a bottom layer 53comprises a homogeneous mixture of platinum on alumina (Pt/Al₂O₃) forAMOx in combination with the rhodium on ceria (Rh/CeO₂) N₂O catalystlocated on a flow-through carrier/substrate 52.

FIG. 6 depicts an exemplary zoned composite 60 of a SCR/AMOx catalystand a N₂O catalyst where a front zone 66 comprises a Cu-zeolite catalystfor SCR on the flow-through carrier/substrate 62, and a rear zone 65comprises a homogeneous mixture of platinum on alumina (Pt/Al₂O₃) forAMOx in combination with the rhodium on ceria (Rh/CeO₂) N₂O catalyst.

FIGS. 7A-7D depict exemplary layered and zoned composites of theSCR/AMOx+N₂O catalyst. In FIG. 7A, composite 70A comprises aflow-through carrier/substrate 72 on which is deposited: a front zone 76comprising a Cu-zeolite catalyst for SCR and a layered rear zone havinga bottom layer 74 comprising a Rh/CeO₂ N₂O catalyst and a top layercomprising a Pt/Al₂O₃ AMOx catalyst. In FIG. 7B, composite 70B comprisesa flow-through carrier/substrate 72 on which is deposited: a top layer76 of a front zone comprising a Cu-zeolite catalyst for SCR and a bottomlayer 77 of the front zone comprising a Pt/Al₂O₃ AMOx catalyst and arear zone 75 comprising a Rh/CeO₂ N₂O catalyst. In FIG. 7C, composite70C comprises a flow-through carrier/substrate 72 on which is deposited:a top layer 76 comprising a Cu-zeolite catalyst for SCR and a bottomlayer having a front zone 75 comprising a Pt/Al₂O₃ AMOx catalyst and arear zone 74 comprising a Rh/CeO₂ N₂O catalyst. In FIG. 7D, composite70D comprises a flow-through carrier/substrate 72 on which is deposited:a top layer and front zone 76 comprising a Cu-zeolite catalyst for SCRand a rear zoned bottom layer 73 comprising a homogeneous mixture of thePt/Al₂O₃ AMOx catalyst in combination with the Rh/CeO₂N₂O catalyst.

FIG. 8 depicts an exemplary SCRoF with N₂O function composite 80, wherean upstream side 81 of a wall-flow filter 83 comprises a layer 86 onand/or in the filter walls comprising a Cu-zeolite which is an SCRcatalyst, and a downstream side 89 of the filter 83 comprises a zone ofN₂O catalyst 85. It is expected that a urea feed would supply the SCRoF80.

FIG. 9A depicts an exemplary composite 90 of a wall-flow filter 93 withN₂O catalyst, where an upstream side 91 of the wall-flow filter 93comprises a layer 96 comprising Cu—O for hydrogen sulfide (H₂S) trappingand/or a Cu-zeolite which provides SCR functionality, and a downstreamside 99 of the filter 93 comprises a zone 95 comprising platinum and/orpalladium on alumina to provide CO and/or HC oxidation in combinationwith the Rh/CeO₂ N₂O catalyst.

FIG. 9B depicts another exemplary composite 100 of a wall-flow filter103 with N₂O catalyst, where an upstream side 101 of the wall-flowfilter 103 comprises a front zone 107 comprising Cu—O for hydrogensulfide (H₂S) trapping, and a downstream side 109 of the filter 103comprises a zone 105 that is a homogeneous mixture comprising platinumand/or palladium on alumina to provide CO and/or HC oxidation incombination with the Rh/CeO₂ N₂O catalyst.

FIG. 10A depicts an exemplary composite 110 of a gasoline particulatefilter 113 having a TWC catalyst and a N₂O catalyst 110, where anupstream side 111 of a wall-flow filter suitable for capturing gasolineparticulates 113 comprises a front zone 115 comprising a homogeneousmixture of a TWC catalyst comprising palladium on alumina and an oxygenstorage component (OSC) such as a ceria-zirconia composite, and adownstream side 119 of the filter 113 comprises a zone 117 comprisingthe Rh/CeO₂ N₂O catalyst.

FIG. 10B depicts another exemplary gasoline particulate filter havingTWC catalyst and N₂O catalyst 120, where an upstream side 121 of awall-flow filter suitable for capturing gasoline particulates 123comprises a layer 126 comprising a homogeneous mixture of a TWC catalystcomprising palladium on alumina and an oxygen storage component (OSC)such as a ceria-zirconia composite, and a downstream side 129 of thefilter 123 comprises a layer 124 comprising the Rh/CeO₂ N₂O catalyst.

Before describing several exemplary embodiments of the invention, it isto be understood that the invention is not limited to the details ofconstruction or process steps set forth in the following description.The invention is capable of other embodiments and of being practiced invarious ways. In the following, preferred designs are provided,including such combinations as recited used alone or in unlimitedcombinations, the uses for which include catalysts, systems, and methodsof other aspects of the present invention.

Specific Embodiments

Various embodiments are listed below. It will be understood that theembodiments listed below may be combined with all aspects and otherembodiments in accordance with the scope of the invention.

Embodiment 1

A nitrous oxide (N₂O) removal catalyst article for an exhaust stream ofan internal combustion engine comprising: a N₂O removal catalyticmaterial on a substrate, the catalytic material comprising a rhodium(Rh) component supported on a ceria-based support, wherein the catalyticmaterial has a H₂-consumption peak of 100° C. or less as measured byhydrogen temperature-programmed reduction (H₂-TPR) and is effective todecompose nitrous oxide (N₂O) to nitrogen (N₂) and oxygen (O₂) and/or toreduce N₂O to N₂ and water (H₂O) and/or (CO₂) under conditions of theexhaust stream.

Embodiment 2

The N₂O removal catalyst composite of embodiment 1, wherein theH₂-consumption peak after aging at 750° C. for 20 hours with 10 volume %water in air occurs at a lower temperature than the temperature of theH₂-consumption peak of fresh catalytic material.

Embodiment 3

The N₂O removal catalyst composite of any of embodiments 1-2, whereinN₂O removal activity of the catalytic material after aging at 750° C.for 20 hours with 10 weight % water is higher than N₂O removal activityof fresh catalytic material.

Embodiment 4

The N₂O removal catalyst composite of any of embodiments 1-3, whereinthe ceria-based support maintains about 90 to about 100% of its porevolume after aging at 750° C. for 20 hours with 10 volume % water inair.

Embodiment 5

The N₂O removal catalyst composite of any of embodiments 1-4, whereinthe ceria-based support comprises 90 to about 100 weight % CeO₂ and hasa pore volume that is at least about 0.20 cm³/g.

Embodiment 6

The N₂O removal catalyst composite of any of embodiments 1-5, whereinthe ceria-based support comprises a fresh surface area that is in therange of about 40 to about 200 m²/g.

Embodiment 7

The N₂O removal catalyst composite of any of embodiments 1-6, whereinthe ceria-based support comprises a surface area that is in the range ofabout 20 to about 140 m²/g after aging at 750° C. for 20 hours with 10weight % water in air.

Embodiment 8

The N₂O removal catalyst composite of any of embodiments 1-7, whereinthe ceria has an average crystallite size in the range of about 3 toabout 20 nm measured by x-ray diffraction (XRD).

Embodiment 9

The N₂O removal catalyst composite of any of embodiments 1-8, whereinthe ceria-based support comprises: an x-ray diffraction averagecrystallite size ratio of aged to fresh material of about 2.5 or less,where aging is conducted at 750° C. for 20 hours with 10% H₂O in air.

Embodiment 10

The N₂O removal catalyst composite of any of embodiments 1-9, whereinthe ceria-based support further comprises a promoter comprising yttria,praseodymia, samaria, gadolinia, zirconia, or silica.

Embodiment 11

The N₂O removal catalyst composite of any of embodiments 1-10, whereinthe ceria-based support comprises ceria in an amount in the range ofabout 56% to about 100% by weight of the support on an oxide basis.

Embodiment 12

The N₂O removal catalyst composite of any of embodiments 1-11, whereinthe rhodium component is present on the support in an amount in therange of 0.01 to 5% by weight of the ceria-based support (including therhodium component).

Embodiment 13

The N₂O removal catalyst composite of any of embodiments 1-12, whereinthe rhodium component is present in an amount of about 0.04 to 3% byweight of the ceria-based support (including the rhodium component).

Embodiment 14

The N₂O removal catalyst composite of any of embodiments 1-13, whereinthe rhodium component has an average crystallite size of about 3 toabout 20 nm.

Embodiment 15

The N₂O removal catalyst composite of any of embodiments 1-14, whereinthe rhodium component is loaded on the ceria-based support in an amountin the range of 1 to about 105 g/ft³.

Embodiment 16

The N₂O removal catalyst composite of any of embodiments 1-15, whereinthe catalytic material further comprises an additional metal component.

Embodiment 17

The N₂O removal catalyst composite of embodiment 16, wherein theadditional metal component comprises platinum (Pt), palladium (Pd),silver (Au), copper (Cu), or combinations thereof.

Embodiment 18

The N₂O removal catalyst composite of embodiment 16, wherein thecatalytic material further comprises a metal oxide for promoting the Rhand/or additional metal component(s).

Embodiment 19

The N₂O removal catalyst composite of embodiment 18, wherein theadditional metal oxide comprises ceria, yttria, samaria, or gadolinia.

Embodiment 20

The N₂O removal catalyst composite of any of embodiments 1-19, whereinthe substrate comprises a monolithic substrate.

Embodiment 21

The N₂O removal catalyst composite of any of embodiments 1-19, whereinthe substrate comprises a wall-flow filter.

Embodiment 22

A catalyst composite for an exhaust stream of an internal combustionengine comprising: a N₂O removal catalytic material in a washcoat on asubstrate, the catalytic material comprising a rhodium (Rh) componentsupported on a ceria-based support and is effective to convert nitrousoxide (N₂O) under conditions of the exhaust stream, wherein theceria-based support comprises: about 90 to about 100 weight % CeO₂; apore volume that is in the range of about 0.20 to 0.40 cm³/g; a freshsurface area that is in the range of about 40 to about 200 m²/g; and anaged surface area that is in the range of about 20 to about 140 m²/gafter aging at 750° C. for 20 hours with 10 weight % water in air.

Embodiment 23

The catalyst composite of embodiment 22, wherein the catalytic materialfurther includes a precious group metal on a high surface arearefractory metal oxide support that is effective to oxidize hydrocarbonsand/or carbon monoxide under conditions of the exhaust stream.

Embodiment 24

An emissions treatment system for treatment of an internal combustionengine exhaust stream including hydrocarbons, carbon monoxide, andnitrogen oxides, the emission treatment system comprising: an exhaustconduit in fluid communication with the internal combustion engine viaan exhaust manifold; a treatment catalyst; and the N₂O removal catalystcomposite according to any one of embodiments 1-23.

Embodiment 25

The emissions treatment system of embodiment 24, wherein the treatmentcatalyst comprises a nitrogen oxide treatment catalyst, which comprises:a three-way conversion (TWC) catalyst, a lean NOx trap (LNT), and/or aselective catalytic reduction (SCR) catalyst.

Embodiment 26

The emissions treatment system of either embodiment 24 or 25, whereinthe N₂O removal catalyst composite is located downstream of the nitrogenoxide treatment catalyst.

Embodiment 27

The emissions treatment system of either embodiment 24 or 25, whereinthe system is zoned and the nitrogen oxides treatment catalyst is in afront zone and the N₂O removal catalyst composite is in a back zone.

Embodiment 28

The emissions treatment system of either of embodiment 24 or 25, whereinthe system is layered and the nitrogen oxides treatment catalyst is inan outer layer and the N₂O removal catalytic material of the catalystcomposite is in an inner layer.

Embodiment 29

The emissions treatment system of either of embodiment 24 or 25, whereinthe system is layered and the nitrogen oxides treatment catalyst is inan inner layer and the N₂O removal catalytic material of the N₂Ocatalyst composite is in an outer layer.

Embodiment 30

A method for treating exhaust gases comprising contacting a gaseousstream comprising hydrocarbons, carbon monoxide, and nitrogen oxideswith the N₂O removal catalyst composite according to any of embodiments1-23.

Embodiment 31

The method of embodiment 30, wherein N₂O removal activity of thecatalytic material after aging at 750° C. for 20 hours with 10 weight %water is higher than N₂O removal activity of fresh catalytic material.

Embodiment 32

A method of making a nitrous oxide (N₂O) removal catalyst composite, themethod comprising: depositing a rhodium component (e.g., Rh reagent)onto a fresh ceria-based support having a pore volume that is at leastabout 0.20 cm³/g and forming a washcoat therefrom; coating a substratecomprising a flow-through monolith or a wall-flow filter with thewashcoat to form a coated substrate; and calcining the coated substrateat an elevated temperature.

Embodiment 33

The method of embodiment 32, wherein the calcining step is underconditions of 750° C. for 20 hours with 10 volume % water in air.

Embodiment 34

The method of embodiment 32, wherein the ceria-containing support ispre-calcined at 700° C. for 2 hours before Rh deposition.

EXAMPLES

The following non-limiting examples shall serve to illustrate thevarious embodiments of the present invention. In each of the examples,the substrate was cordierite.

Example 1: Preparation

Method 1.1. Rh nitrate solution was impregnated onto a powder ceriasupport with incipient wetness technique to achieve a desirable Rh metalloading. The Rh-impregnated support was then dispersed in deionizedwater to achieve 30% solid and the slurry pH was adjusted with HNO₃ to4. The resulting slurry was milled and dried while stirring. The driedslurry was calcined at 500° C. for 2 hours in air and then crushed andsieved to 250 to 500 μm for reactor testing.

Method 1.2. The procedure was the same as in Method 1.1 with oneexception; the final calcination temperature was 800° C.

Method 1.3. A support powder was first pre-calcined at 800° C. for 2hours in air before metal impregnation. The rest of the procedures werethe same as Method 1.1.

Method 1.4. A support powder was first dispersed in deionized water toreach 30% solid content. The slurry was then milled, and Rh nitratesolution (or a nitrate solution of Rh+another metal) was added to themilled slurry. The slurry was dried while stirring. The dried slurry wascalcined at 500° C. for 2 hours in air and then crushed and sieved to250-500 μm for reactor testing.

Method 1.5. Rh nitrate solution was impregnated onto a powder supportwith the incipient wetness technique to achieve a desirable Rh metalloading. The Rh/support powder was dried and calcined at 500° C. for 2hours. The resulting material was impregnated with another metal (suchas Pt or Pd) using the same methodology and then dried and calcined at500° C. for 2 hours. The bimetallic catalyst powder was then dispersedin deionized water to achieve 30% solid and the slurry pH was adjustedwith HNO₃ to 4. The resulting slurry was milled and dried whilestirring. The dried slurry was calcined at 500° C. for 2 hours in airand then crushed and sieved to 250-500 μm for reactor testing.

Method 1.6. For supported Au/Rh and Ir/Rh catalyst, the procedures weresimilar to Method 1.5. However, a washing step was added before theslurry step using CO₂ saturated deionized water to remove Cl ions.

Method 1.7. Rh and another metal (or oxide) were co-impregnated onto aCeO₂ support using the incipient wetness technique. The rest of thepreparation procedures were the same as Method 1.1.

Method 1.8. A cerium oxide support was modified by a rare earth metaloxide (10% by weight) by impregnating a rare earth metal nitratesolution onto the support using the incipient wetness technique. Thismodified cerium oxide powder was dried at 110° C. for 2 hours and thencalcined at 500° C. for 2 hours. This resulting powder was thenimpregnated with Rh with the same methodology to achieve a desirable Rhmetal loading. The resulting powder was then dispersed in deionizedwater to achieve 30% solid and the slurry pH was adjusted with HNO₃ to4. The resulting slurry was milled and dried while stirring. The driedslurry was calcined at 500° C. for 2 hours in air and then crushed andsieved to 250-500 μm for reactor testing.

Method 1.9. The order of impregnation for Rh and another metal in Method1.8 was reversed. The rest of the procedures were the same.

Example 2: Ceria-Based Support Characteristics

Table 1 summarizes some physical and chemical characterization data,including BET surface area, pore volume, average pore radius and X-raydiffraction data collected on a number of CeO₂ support materials as wellas H₂ temperature programmed reduction data obtained on resultingRh/CeO₂ catalysts. Conditions for aging of all CeO₂ materials were: 750°C. for 20 hours with 10% H₂O in air. Rh/CeO₂ catalysts tested contained1% Rh on CeO₂ by weight.

Hydrogen temperature-programmed reduction (H₂-TPR) was carried out on aMicromeritics AutoChem Series Instrument. Prior to the test, each samplewas pretreated under a flow of 4% O₂ balanced with He at 500° C. for 30min and then cooled down to ambient temperature. The TPR experiment wasperformed in 1% H₂ balanced with N₂ at a gas flow rate of 50 cc/min andthe temperature was ramped from 20 to 900° C. at a ramping rate of 10°C./min.

TABLE 1 % Change Rh/CeO₂ BET % Change in pore XRD XRD H₂-TPR surfacePore Pore in BET volume Crystallite size ratio 1^(st) peak Supportmaterial area volume radius (rel. to (rel. to size (Aged/ Temp. & Agingcondition (m²/g) (cm³/g) (nm) fresh) fresh) (nm) Fresh) (° C.) CeO₂ (A),Fresh 144 0.31 3.5 N/A N/A 6.2 91 CeO₂ (B), Fresh 177 0.07 2.0 N/A N/A6.7 125 CeO₂ (C), Fresh 159 0.21 2.7 N/A N/A 6.5 99 CeO₂ (D), Fresh 1310.32 3.8 N/A N/A 7.3 85 CeO₂ (A), Aged 77 0.30 5.7 −47 −3 12.9 2.1 85CeO₂ (B), Aged 13 0.04 4.0 −93 −43 39.8 5.9 88 CeO₂ (C), Aged 54 0.195.6 −66 −10 15.5 2.4 95 CeO₂ (D), Aged 70 0.31 6.7 −47 −3 14.9 2.0 71

Regarding BET surface area, CeO₂(A) and CeO₂(D) have the least decreaseafter 750° C./20 h aging. As to pore volume, CeO₂(A) and CeO₂(D) havethe largest pore volume and the least change after aging (−3%). CeO₂(A)and CeO₂(D) have the least crystal agglomeration after aging. The H₂-TPRresults show that Rh/CeO₂(D) is easiest to reduce (1^(st) peak at 85° C.fresh and 71° C. aged) and also has the highest H₂ consumption.

Example 3

The effect of CeO₂ material type and thermal treatment was analyzed forvarious N₂O catalysts having a composition of 1% weight Rh/CeO₂ usingthe supports described in Example 2. Table 2 provides a summary of thecatalyst powders prepared.

TABLE 2 Rh Prepa- Catalyst Loading EXAMPLE 2 Support ration ID (%)Support Type composition Method 3.1 1 CeO₂ (A) 100% CeO₂ 1.1 3.2 1 CeO₂(B) 100% CeO₂ 1.1 3.3 1 CeO₂ (C) 100% CeO₂ 1.1 3.4 1 CeO₂ (D) 100% CeO₂1.1 3.5 1 CeO₂ (A) 100% CeO₂ 1.2 3.6 1 CeO₂ (A) 100% CeO₂ 1.3 3.7 1 CeO₂(B) 100% CeO₂ 1.2 3.8 1 CeO₂ (C) 100% CeO₂ 1.2 3.9 1 CeO₂ (D) 100% CeO₂1.4

Tables 3A, 3B, and 3C provide the N₂O removal (specificallydecomposition) activity, in terms of N₂O conversion, of the catalystsunder various simulated feed conditions, as freshly-produced and after20 hours aging at 750° C., comparing the N₂O activities of a group ofRh/CeO₂ catalysts as a function of CeO₂ material. The N₂O decompositionactivities were measured with a high-throughput reactor capable oftesting multiple samples in a single test run. The basic reaction feedcontained 200 ppm N₂O, 5 wt.-% CO₂ and balance N₂. In separate tests, 5wt. % O₂ or 5 wt. % O₂+5% H₂O by volume were added to the basic feed,respectively. The activity was measured at constant temperatures of 250,300, 350, 400 and 450° C. For each run, 0.2 grams of sample was usedwith a flow rate of 50 L/min, which is equivalent a monolithic GHSV of30,000 h⁻¹ with 2 g/in³ washcoat loading. Each catalyst was testedas-fresh (as-is) and aged (750° C. for 20 hours with 10 wt.-% H₂O inair) sample. N₂O only refers to a dry feed, N₂O+O₂ refers to anair-containing feed, and N₂O+O₂+H₂O refers to a wet air feed (nominal %water by volume).

TABLE 3A N₂O only Catalyst 300° C. 350° C. 400° C. 450° C. FRESH 3.1 1125 70 91 3.2 4 13 43 73 3.3 3 11 40 73 3.4 51 97 100 100 3.5 26 75 95 983.6 55 92 97 97 3.7 4 18 55 82 3.8 13 43 85 97 3.9 52 89 99 99 750° C.AGED 3.1 9 29 80 96 3.2 1 5 24 55 3.3 1 12 50 80 3.4 74 100 100 100 3.538 88 99 100 3.6 34 71 85 90 3.7 1 11 44 74 3.8 6 31 71 90 3.9 56 98 100100

TABLE 3B N₂O + O₂ Catalyst 300° C. 350° C. 400° C. 450° C. FRESH 3.1 420 41 77 3.2 1 8 25 56 3.3 0 6 21 53 3.4 34 91 99 100 3.5 16 65 88 953.6 38 87 95 97 3.7 2 4 37 64 3.8 7 17 67 88 3.9 30 89 97 99 750° C.AGED 3.1 7 13 57 87 3.2 0 1 12 36 3.3 1 3 32 65 3.4 51 96 99 100 3.5 2481 96 98 3.6 22 61 79 84 3.7 1 3 26 56 3.8 4 7 53 80 3.9 37 93 100 100

TABLE 3C N₂O + O₂ + H₂O Catalyst 300° C. 350° C. 400° C. 450° C. FRESH3.1 0 2 15 37 3.2 0 0 6 27 3.3 0 0 4 20 3.4 0 4 24 60 3.5 1 4 29 59 3.61 8 38 78 3.7 0 1 10 23 3.8 0 0 19 44 3.9 0 2 30 67 750° C. AGED 3.1 1 229 47 3.2 0 1 7 9 3.3 0 1 15 23 3.4 0 4 36 77 3.5 1 3 22 58 3.6 0 3 1846 3.7 0 1 9 16 3.8 0 1 16 33 3.9 0 4 64 92

Catalysts 3.1 to 3.4 are Rh supported on different CeO₂ materials madewith the same preparation method, as summarized in Table 2 of Example 3.Catalysts 3.1 and 3.4, supported on CeO₂(A) and CeO₂(D), respectively,are the most active Rh/CeO₂ catalysts. Surprisingly, the aged activitiesof these two catalysts are higher than their fresh activities. Catalysts3.2 and 3.3, supported on CeO₂(B) and CeO₂(C), respectively, are lessactive for N₂O decomposition than Catalysts 3.1 and 3.4 under all testconditions. In contrast to Catalysts 3.1 and 3.4, the aged N₂Oconversions of Catalysts 3.2 and 3.3 are significantly lower than theirfresh catalysts.

Tables 3A, 3B, and 3C also show the effect of thermal treatment ofRh/CeO₂ catalysts as a part of catalyst preparation step. A hightemperature (800° C./2 hours) calcination treatment of Rh/CeO₂ increasesthe N₂O conversion on all Rh/CeO₂ catalysts and under all test andcatalyst aging conditions. For example, high temperature calcinedRh/CeO₂(A)—Catalyst 3.5 has a N₂O conversion of 55% at 450° C. afteraging tested with 5% H₂O by volume in feed, which compares favorablywith 47% of its parent catalyst (Catalyst 3.1) tested under the sameconditions. The conversion increase on the fresh activity is even morepronounced (59% on Catalyst 3.5 vs. 37% on Catalyst 3.1 at 450° C.). ForRh/CeO₂(B) and Rh/CeO₂(C), the conversion increases on aged catalysts at450° C. are 16% (Catalyst 3.7) from 9% (Catalyst 3.2) and 33% (Catalyst3.8) from 23% (Catalyst 3.3), respectively. Thermal calcination of theCeO₂ support (800° C./2 hours) before Rh impregnation also promotes theN₂O activity. Catalyst 3.6, its support was pre-calcined before Rhimpregnation, is much more active for all fresh N₂O activity tests thanCatalyst 3.1; its conversion is 78% at 450° C. tested with a wet feedvs. 37% for Catalyst 3.1 under the same conditions. In addition,catalyst preparation method also has a significant impact on N₂Oactivity. For example, Catalyst 3.9 (Rh supported CeO₂(D) made with Rhsolution added in the CeO₂(D) slurry) has a N₂O conversion of 92% at450° C. after aging tested with 5% H₂O by volume, whereas aged Catalyst3.4 (Rh supported on CeO₂(D) made with the incipient wetness technique)has a conversion of 60% under the same conditions.

Example 4

The effect of modifying CeO₂ with rare earth (RE) metal oxides wasanalyzed for various N₂O decomposition catalysts having a composition of1% Rh/-RE-CeO₂. Table 4 provides a summary of the catalyst powdersprepared.

TABLE 4 Rh Prepa- Catalyst Loading EXAMPLE 2 Support ration ID (%)Support Type composition Method 3.1 1 CeO₂ (A) 100% CeO₂ 1.1 4.1 1Y₂O₃/CeO₂ (A) 10% Y₂O₃/CeO₂ (A) 1.8 4.2 1 La₂O₃/CeO₂ (A) 10%La₂O₃/CeO₂(A) 1.8 4.3 1 Pr₂O₃/CeO₂ (A) 10% Pr₂O₃/CeO₂ (A) 1.8 4.4 1Nd₂O₃/CeO₂ (A) 10% Nd₂O₃/CeO₂ (A) 1.8 4.5 1 Sm₂O₃/CeO₂ (A) 10%Sm₂O₃/CeO₂ (A) 1.8 4.6 1 Gd₂O₃/CeO₂ (A) 10% Gd₂O₃/CeO₂ (A) 1.8 4.7 1CeO₂/CeO₂ (A) 10% CeO₂/CeO₂(A) 1.8

Tables 5A, 5B, and 5C provide the N₂O removal activity, in terms of N₂Oconversion, of the catalysts under various simulated feed conditions, asfreshly-produced and after 20 hours aging at 750° C., comparing the N₂Oactivities of a group of Rh/CeO₂ catalysts as a function of RE-CeO₂material. N₂O only refers to a dry feed, N₂O+O₂ refers to anair-containing feed, and N₂O+O₂+H₂O refers to a wet air feed (nominal 5%water by volume).

TABLE 5A N₂O only Catalyst 300° C. 350° C. 400° C. 450° C. FRESH 3.1 1125 70 91 4.1 20 67 97 100 4.2 3 14 43 82 4.3 14 53 92 100 4.4 17 64 94100 4.5 27 76 97 100 4.6 23 69 95 100 4.7 19 43 90 98 750° C. AGED 3.1 929 80 96 4.1 0 1 6 23 4.2 0 1 8 33 4.3 3 6 48 87 4.4 2 6 45 83 4.5 2 757 91 4.6 2 5 39 77 4.7 8 29 78 92

TABLE 5B N₂O + O₂ Catalyst 300° C. 350° C. 400° C. 450° C. FRESH 3.1 420 41 77 4.1 9 58 80 98 4.2 0 9 32 64 4.3 5 41 72 95 4.4 7 51 80 96 4.514 67 86 98 4.6 11 61 83 96 4.7 8 42 69 93 750° C. AGED 3.1 7 13 57 874.1 0 0 2 13 4.2 0 1 4 20 4.3 2 9 16 65 4.4 2 10 16 61 4.5 2 12 19 724.6 1 8 13 54 4.7 6 10 57 84

TABLE 5C N₂O + O₂ + H₂O Catalyst 300° C. 350° C. 400° C. 450° C. FRESH3.1 0 2 15 37 4.1 0 0 5 51 4.2 0 0 1 14 4.3 0 0 5 41 4.4 0 0 6 47 4.5 01 8 57 4.6 0 0 7 48 4.7 0 1 18 41 750° C. AGED 3.1 1 2 29 47 4.1 0 0 0 14.2 0 0 1 2 4.3 1 1 3 14 4.4 0 0 2 11 4.5 1 1 3 15 4.6 0 1 2 9 4.7 1 222 43

Catalyst 3.1 is a “parent” catalyst, which is one containing noadditional added rare earth, compared to Catalysts 4.1-4.7. Rare earthmodification of CeO₂ by impregnation increases the fresh N₂O conversion.Performance is lower than the parent catalyst after aging.

Example 5

The effect of modifying Rh-CeO₂ by addition of platinum group or othermetals (ME) was analyzed for various N₂O removal catalysts havingvarious compositions of n % ME/1% Rh/CeO₂. Table 6 provides a summary ofthe catalyst powders prepared.

TABLE 6 Rh Other Prepa- Catalyst Loading metal EXAMPLE 2 Support rationID (%) (n %) Support Type composition Method 3.9 1 CeO₂ (D) 100% CeO₂1.4 5.1 1 2% Cu CeO₂ (D) 100% CeO₂ 1.7 5.2 1 0.2% Ru CeO₂ (D) 100% CeO₂1.7 5.3 1 0.2% Pd CeO₂ (D) 100% CeO₂ 1.7 5.4 1 0.2% Ag CeO₂ (D) 100%CeO₂ 1.7 5.5 1 0.2% Pt CeO₂ (D) 100% CeO₂ 1.7 5.6 1 0.2% Ir CeO₂ (D)100% CeO₂ 1.6 5.7 1 0.02% Au CeO₂ (D) 100% CeO₂ 1.6 5.8 1 0.2% Au CeO₂(D) 100% CeO₂ 1.6 5.9 1 0.02% Pd CeO₂ (D) 100% CeO₂ 1.5 5.10 1 0.2% PdCeO₂ (D) 100% CeO₂ 1.5 5.11 1 0.02% Pt CeO₂ (D) 100% CeO₂ 1.5 5.12 10.2% Pt CeO₂ (D) 100% CeO₂ 1.9 5.13 1 0.2% Pd CeO₂ (D) 100% CeO₂ 1.95.14 1 1% Pd CeO₂ (D) 100% CeO₂ 1.9 5.15 1 1% Pt CeO₂ (D) 100% CeO₂ 1.9

Tables 7A, 7B, and 7C provide the N₂O removal activity, in terms of N₂Oconversion, of the catalysts under various simulated feed conditions, asfreshly-produced and after 20 hours aging at 750° C., comparing the N₂Oactivities of a group of Rh/CeO₂ catalysts as a function of secondarymetal addition. N₂O only refers to a dry feed, N₂O+O₂ refers to anair-containing feed, and N₂O+O₂+H₂O refers to a wet air feed (nominal 5wt.-% water).

TABLE 7A N₂O only Catalyst 300° C. 350° C. 400° C. 450° C. FRESH 3.9 5289 99 99 5.1 44 90 100 100 5.2 47 92 100 100 5.3 56 96 100 100 5.4 52 94100 100 5.5 41 85 100 100 5.6 34 84 100 100 5.7 46 91 100 100 5.8 30 83100 100 5.9 86 100 100 100 5.10 52 96 100 100 5.11 56 95 100 100 5.12 5995 100 100 5.13 53 97 100 100 5.14 61 98 100 100 5.15 54 98 100 100 750°C. AGED 3.9 56 98 100 100 5.1 27 71 100 100 5.2 47 91 100 100 5.3 45 94100 100 5.4 74 100 100 100 5.5 38 92 100 99 5.6 43 93 96 96 5.7 52 94 9797 5.8 43 95 98 98 5.9 71 100 100 100 5.10 44 93 99 99 5.11 67 98 99 995.12 62 99 100 100 5.13 44 94 100 100 5.14 38 91 100 100 5.15 39 89 9696

TABLE 7B N₂O + O₂ Catalyst 300° C. 350° C. 400° C. 450° C. FRESH 3.9 3089 97 99 5.1 24 83 98 100 5.2 33 92 98 100 5.3 34 94 100 100 5.4 33 9499 100 5.5 26 83 97 100 5.6 20 73 97 100 5.7 28 86 99 100 5.8 21 79 98100 5.9 61 99 100 100 5.10 35 95 100 100 5.11 35 91 99 100 5.12 38 93 99100 5.13 35 96 100 100 5.14 43 95 100 100 5.15 35 95 100 100 750° C.AGED 3.9 37 93 100 100 5.1 18 59 92 100 5.2 32 83 98 99 5.3 32 90 100100 5.4 50 96 100 100 5.5 28 84 99 99 5.6 31 84 96 96 5.7 36 90 97 975.8 31 84 98 98 5.9 52 98 100 100 5.10 33 87 98 99 5.11 47 96 99 99 5.1244 97 100 100 5.13 33 88 100 100 5.14 31 82 99 100 5.15 32 82 95 96

TABLE 7C N₂O + O₂ + H₂O Catalyst 300° C. 350° C. 400° C. 450° C. FRESH3.9 0 2 30 67 5.1 0 1 20 60 5.2 0 2 26 70 5.3 0 2 31 69 5.4 0 2 24 625.5 0 2 28 62 5.6 0 2 26 49 5.7 0 3 38 61 5.8 0 2 30 56 5.9 0 5 61 845.10 0 3 46 70 5.11 1 5 66 85 5.12 0 6 62 81 5.13 0 3 47 72 5.14 0 3 3874 5.15 0 4 44 69 750° C. AGED 3.9 0 4 64 92 5.1 0 0 38 65 5.2 1 2 39 795.3 1 3 60 91 5.4 1 3 58 88 5.5 0 3 62 90 5.6 0 3 49 79 5.7 0 4 55 835.8 0 4 54 82 5.9 0 4 56 90 5.10 0 3 50 83 5.11 0 5 67 91 5.12 0 5 68 935.13 0 4 56 88 5.14 0 3 49 88 5.15 0 4 58 88

The “parent” catalyst here is Catalyst 3.9 [Rh/CeO₂(D)], which containsRh only. Incorporating some Pd, Pt and Ru (especially Pd and Pt) intoCatalyst 3.9 significantly increases the N₂O conversion on freshcatalysts. The improved catalysts are Catalysts 5.9-5.15, Catalysts5.2-5.3. Some Pd- or Pt-modified catalysts are even slightly more activethan or comparable to the reference after aging (Catalysts 5.6 and5.11-5.12).

Example 6

A N₂O catalyst composite was prepared using a catalyst according to 3.1.Rh nitrate solution was impregnated onto a powder support (CeO₂) withthe incipient wetness technique to achieve a desirable Rh metal loading.The resulting powder was dried at 110° C. for 5 hours then calcined at550° C. for 2 hours to form catalytic material, a portion of which wasretained for testing.

The catalytic material/calcined powder was then dispersed in water andmilled in a continuous mill to D₉₀<12 μm (90% particles with a diameterless than 12 μm) to form a washcoat. The slurry of the pH was adjustedusing acetic acid to pH=5, and the solid content of the slurry was about37%. A monolith substrate, with a cell density of 600 cells/in² and 0.10mm wall thickness, was immersed into the slurry for a few seconds.Compressed air was blown through the coated monolith to remove excesscoating. The coated sample was dried in flowing air at about 200° C. for20 minutes and then calcined at 550° C. for 2 hours to form a catalyticcomposite. The sample after calcination contained 35 g/ft³ Rh and 2g/in³ support (CeO₂).

Example 7 (Comparative)

A comparative N₂O catalyst composite was prepared using Rh on alumina(Al₂O₃). Rh nitrate solution was impregnated onto a powder support(Al₂O₃) with the incipient wetness technique to achieve a desirable Rhmetal loading. The resulting powder was dried at 110° C. for 5 hoursthen calcined at 550° C. for 2 hours. The calcined powder was thendispersed in water and milled in a continuous mill to D₉₀<12 μm (90%particles with a diameter less than 12 μm). The slurry of the pH wasadjusted using acetic acid to pH=5, and the solid content of the slurrywas about 37%. A monolith substrate, with a cell density of 600cells/in² and 0.10 mm wall thickness, was immersed into the slurry for afew seconds. Compressed air was blown through the coated monolith toremove excess coating. The coated sample was dried in flowing air atabout 200° C. for 20 minutes and then calcined at 550° C. for 2 hours.The sample after calcination contained 35 g/ft³ Rh and 2 g/in³ support(Al₂O₃).

Example 8

The catalytic material/calcined powder of Examples 6-7 were tested in ahigh throughput reactor for N₂O decomposition activity.

FIG. 11 provides N₂O activities over the catalytic material of Example6. Over either fresh or aged catalyst, H₂O has a strong impact on N₂Oconversion. However, the aged catalyst is unexpectedly more active thanthe fresh, especially with a wet feed.

FIG. 12 shows N₂O activities over the catalytic material of ComparativeExample 7. Although the fresh catalyst is more active than the catalystshown in FIG. 11, its activity is severely deactivated by aging. The N₂Oconversion is near zero after aging tested with a wet feed.

Example 9 (Testing)

The monolith catalyst of Examples 6-7 were tested as follows. The N₂Odecomposition activities were measured with a laboratory, steady-stateflow reactor at GHSV=30,000 h⁻¹. The sample was a 1 inch (diameter)×1inch (length) monolith core positioned in the rear part of an electricalfurnace. The basic reaction feed contained 200 ppm N₂O, 5 wt. % CO₂ andbalance N₂. In separate tests, 5 wt. % O₂ or 5 wt. % O₂+5% H₂O by volumewere added to the basic feed, respectively. The activity was measuredbetween 200 and 500° C. with a temperature ramp at a ramp rate of 15°C./min. The effluent gas composition was measured by a MKS FTIR Analyzer(Model 2030DGB2EVS13T) at a collection speed of 1 Hz. Each sample wastested as fresh (as-is) and aged (750° C. for 20 hours with 10. % H₂O byvolume in air) sample.

FIG. 13 provides N₂O activities over the monolithic catalyst compositeof Example 6. The monolith data shown in FIG. 13 are in generalconsistent with the powder data shown in FIG. 11. However, the promotioneffect by aging is more pronounced on the monolith catalyst.

FIG. 14 provides N₂O activities over the monolithic catalyst ofComparative Example 7. The monolith data shown in FIG. 14 are consistentwith the powder data shown in FIG. 12. Therefore, the powder datapresented in FIGS. 11 and 12 as well as in Tables 3A, 3B, 3C, 5A, 5B,5C, 7A, 7B, and 7C can represent their catalytic performance in themonolith form.

While this invention has been described with an emphasis upon preferredembodiments, it will be obvious to those of ordinary skill in the artthat variations in the preferred devices and methods may be used andthat it is intended that the invention may be practiced otherwise thanas specifically described herein. Accordingly, this invention includesall modifications encompassed within the spirit and scope of theinvention as defined by the claims that follow.

What is claimed:
 1. A nitrous oxide (N₂O) removal catalyst composite foran exhaust stream of an internal combustion engine comprising: a N₂Oremoval catalytic material on a substrate, the catalytic materialcomprising a rhodium (Rh) component supported on a ceria-based support,wherein the catalytic material has a H₂-consumption peak of about 100°C. or less as measured by hydrogen temperature-programmed reduction(H₂-TPR) and is effective to decompose at least a portion of nitrousoxide (N₂O) in the exhaust stream to nitrogen (N₂) and oxygen (O₂) or toreduce at least a portion of the N₂O to N₂ and water, N₂ and carbondioxide (CO₂), or N₂, water, and CO₂ under conditions of the exhauststream, and wherein the ceria-based support comprises ceria in an amountin the range of about 56 to about 100% by weight of the support on anoxide basis.
 2. The N₂O removal catalyst composite of claim 1, whereinthe H₂-consumption peak after aging at 750° C. for 20 hours with 10volume % water in air occurs at a lower temperature than the temperatureof the H₂-consumption peak of fresh catalytic material.
 3. The N₂Oremoval catalyst composite of claim 1, wherein N₂O removal activity ofthe catalytic material after aging at 750° C. for 20 hours with 10volume % water is higher than N₂O removal activity of fresh catalyticmaterial.
 4. The N₂O removal catalyst composite of claim 1, wherein theceria-based support maintains about 90 to about 100% of its pore volumeafter aging at 750° C. for 20 hours with 10 weight % water in air. 5.The N₂O removal catalyst composite of claim 1, wherein the ceria-basedsupport comprises about 90 to about 100 weight % CeO₂ and has a porevolume that is at least about 0.20 cm³/g.
 6. The N₂O removal catalystcomposite of claim 1, wherein the ceria-based support comprises a freshsurface area that is in the range of about 40 to about 200 m²/g.
 7. TheN₂O removal catalyst composite of claim 1, wherein the ceria-basedsupport comprises a surface area that is in the range of about 20 toabout 140 m²/g after aging at 750° C. for 20 hours with 10 weight %water in air.
 8. The N₂O removal catalyst composite of claim 1, whereinthe ceria has a crystallite size in the range of about 3 to about 20 nmmeasured by x-ray diffraction (XRD).
 9. The N₂O removal catalystcomposite of claim 1, wherein the ceria-based support comprise: an x-raydiffraction crystallite size ratio of aged material to fresh material ofabout 2.5 or less, where aging is at 750° C. for 20 hours with 10% H₂Oin air.
 10. The N₂O removal catalyst composite of claim 1, wherein theceria-based support further comprises a promoter comprising yttria,samaria, gadolinia, zirconia, or silica.
 11. The N₂O removal catalystcomposite of claim 1, wherein the rhodium component is present on thesupport in an amount in the range of about 0.01 to about 5% by weight ofthe support.
 12. The N₂O removal catalyst composite of claim 1, whereinthe rhodium component is present in an amount of about 0.04 to about 3%by weight of the support.
 13. The N₂O removal catalyst composite ofclaim 1, wherein the rhodium component has a crystallite size of lessthan about 5 nm.
 14. The N₂O removal catalyst composite of claim 1,wherein the rhodium component is loaded on the substrate in an amount inthe range of about 1 to about 105 g/ft³.
 15. The N₂O removal catalystcomposite of claim 1, wherein the catalytic material further comprisesan additional metal component.
 16. The N₂O removal catalyst composite ofclaim 15, wherein the additional metal component comprises platinum(Pt), palladium (Pd), silver (Au), copper (Cu), or combinations thereof.17. The N₂O removal catalyst composite of claim 15, wherein thecatalytic material further comprises a metal oxide for promoting the Rhand/or additional metal component.
 18. The N₂O removal catalystcomposite of claim 17, wherein the metal oxide comprises ceria,praseodymia, yttria, samaria, or gadolinia.
 19. The N₂O removal catalystcomposite of claim 1, wherein the substrate comprises a monolithicsubstrate.
 20. The N₂O removal catalyst composite of claim 1, whereinthe substrate comprises a wall-flow filter.
 21. The N₂O removal catalystcomposite of claim 1 for an exhaust stream of an internal combustionengine comprising: a N₂O removal catalytic material in a washcoat on asubstrate, the catalytic material comprising a rhodium (Rh) componentsupported on a ceria-based support and is effective to convert nitrousoxide (N₂O) under conditions of the exhaust stream, wherein theceria-based support comprises: about 90 to about 100 weight % CeO₂; apore volume that is in the range of about 0.20 to about 0.40 cm³/g; afresh surface area that is in the range of about 40 to about 200 m²/g;and an aged surface area that is in the range of about 20 to about 140m²/g after aging at 750° C. for 20 hours with 10 weight % water in air.22. An emissions treatment system for treatment of an internalcombustion engine exhaust stream including hydrocarbons, carbonmonoxide, and nitrogen oxides, the emission treatment system comprising:an exhaust conduit in fluid communication with the internal combustionengine via an exhaust manifold; a treatment catalyst; and the N₂Oremoval catalyst composite of claim
 1. 23. The emissions treatmentsystem of claim 22, wherein the treatment catalyst comprises a preciousmetal on a high surface area refractory metal oxide support that iseffective to oxidize hydrocarbons and/or carbon monoxide underconditions of the exhaust stream.
 24. The emissions treatment system ofclaim 23, wherein the treatment catalyst comprises a nitrogen oxidestreatment catalyst selected from the group consisting of a three-wayconversion (TWC) catalyst, a lean NOx trap (LNT), and a SelectiveCatalytic Reduction (SCR) catalyst.
 25. The emissions treatment systemof claim 24, wherein the N₂O removal catalyst composite is locateddownstream of the nitrogen oxides treatment catalyst.
 26. The emissionstreatment system of claim 24, wherein the system is zoned and thenitrogen oxides treatment catalyst is in a front, upstream zone and theN₂O removal catalyst composite is in a back, downstream zone.
 27. Theemissions treatment system of claim 24, wherein the system is layeredand the nitrogen oxides treatment catalyst is in an outer layer and theN₂O removal catalytic material of the catalyst composite is in an innerlayer.
 28. The emissions treatment system of claim 24, wherein thesystem is layered and the nitrogen oxides treatment catalyst is in aninner layer and the N₂O removal catalytic material of the N₂O catalystcomposite is in an outer layer.
 29. A method for treating exhaust gasescomprising contacting a gaseous stream comprising hydrocarbons, carbonmonoxide, and nitrogen oxides with the N₂O removal catalyst composite ofclaim
 1. 30. The method of claim 29, wherein N₂O removal activity of thecatalytic material after aging at 750° C. for 20 hours with 10 weight %water is higher than N₂O removal activity of fresh catalytic material.